Three conditions or objectives control the administration of an anesthetic, namely, to rapidly produce the desired pharmacologic effect (hypnosis, analgesia, etc.); to maintain the desired effect throughout the medical procedure; and to enable the patient to recover quickly from the effect following completion of the procedure.
In order to achieve the objective of rapidly inducing the desired anesthetic effect, the anesthesiologist typically delivers a so called “Loading Dose.” A Loading Dose is a bolus (mg/kg, mg, etc.) of drug that rapidly brings the patient to a desired level of effect. In order to maintain the level of effect the anesthesiologist often uses an infusion pump to deliver a so called “Maintenance Rate.” A Maintenance Rate is a constant infusion rate (μg/kg/min, mg/min, etc.) required to maintain the patient at a certain target, in this embodiment anesthetic, effect. The anesthesiologist may have to titrate this Maintenance Rate during the procedure as the patient's anesthetic needs change. A method that allows for rapidly adjusting the patient's level of effect is desired. Finally, in order to enable the patient to recover quickly from the anesthetic following completion of the procedure, the anesthesiologist attempts to deliver as little drug as needed. This can include tapering down the Maintenance Rate prior to the end of the procedure.
The term “anesthesia” is used herein to refer to the continuum of hypnosis and analgesia, achieved via anesthetic drugs, from anxiolysis through general anesthesia. In producing a level of anesthesia known as conscious sedation, as practiced by endoscopists, the anesthetic(s) is typically delivered through frequent boluses. This technique results in varying depths of anesthesia throughout the procedure. At times the patient may be so heavily anesthetized as to be classified in general anesthesia. At other times the patient may be under-anesthetized and exhibit pain and agitation. A patient responding to pain is uncooperative, making the procedure more difficult. As a result, the clinician tends to err on the over-anesthetized side. In addition to placing the patient at greater risk for adverse events, over-anesthetizing causes the patient's recovery from anesthesia to be much longer. Accordingly, a method is desired that enables the clinician to control the level of anesthesia without over- or under-anesthetizing the patient.
The term “sedation drug” is used herein to refer to the classes of drugs employed by anesthesiologists in inducing sedation including hypnotics and analgesics. Propofol and remifentanil are preferred drugs for sedation, principally due to their rapid onset and offset. However, this rapid action presents additional concerns for someone using an intermittent bolus technique, as typically done by non-anesthesiologists. With a rapid onset/offset more frequent boluses will be required. Consequently, anesthesiologists often use infusion pumps to continuously deliver these rapid action sedation drugs. However, non-anesthesiologists are not familiar with pharmacokinetic (PK) principals, and will have difficulty determining a Loading Dose/Maintenance Rate combination that will both rapidly achieve and maintain the desired level of anesthesia. The Anesthetic Delivery System (ADS) is intended to enable a non-anesthesiologist to safely and effectively use these rapid action anesthetic agents typically reserved for use by anesthesiologists.
What is desired is an algorithm that will allow the clinician to program an ADS with a desired maintenance rate, selected by the clinician to maintain a desired level of anesthesia, and then the ADS automatically calculates the appropriate sized loading dose based on the pharmacokinetics of the chosen sedation drug. The loading dose is then delivered by the ADS to rapidly achieve the level of sedation, immediately followed by a constant infusion of the sedation drug at the maintenance rate, to maintain the level of anesthesia. Moreover, a method is desired where the patient's level of anesthesia is rapidly adjusted, each time the clinician changes the maintenance rate, in response to the patient's changing anesthetic needs. Specifically, what is needed is an ADS that integrates the initiation and maintenance of anesthesia in an equation so that the appropriate sized loading dose may be calculated and administered to rapidly bring the patient's depth of anesthesia to a level maintained by the programmed maintenance rate. Further, when a change in the maintenance rate is requested, the dosage controller (DC) can calculate an incremental loading dose to rapidly achieve the new level of anesthesia.